This invention relates to supported titanium trichloride catalyst systems. It is broadly known to utilize TiCl.sub.3 or even TiCl.sub.4 as a propylene polymerization catalyst. However, such catalysts tend to produce a high proportion of material which is soluble in solvents such as normal hexane. It has been found that the solubles formation can be suppressed by the use of various adjuvants as disclosed, for instance, in German Pat. No. 2,347,577. Unfortunately, suppression of solubles formation also generally suppresses the overall production of polymer.